Description

CREATE CAST defines a new cast. A
cast specifies how to perform a conversion between two data
types. For example:

SELECT CAST(42 AS float8);

converts the integer constant 42 to type float8 by invoking a previously specified function,
in this case float8(int4). (If no
suitable cast has been defined, the conversion fails.)

Two types can be binary coercible,
which means that the conversion can be performed "for free" without invoking any function. This
requires that corresponding values use the same internal
representation. For instance, the types text and varchar are binary
coercible both ways. Binary coercibility is not necessarily a
symmetric relationship. For example, the cast from xml to text can be performed
for free in the present implementation, but the reverse direction
requires a function that performs at least a syntax check. (Two
types that are binary coercible both ways are also referred to as
binary compatible.)

You can define a cast as an I/O
conversion cast using the WITH INOUT
syntax. An I/O conversion cast is performed by invoking the
output function of the source data type, and passing the result
to the input function of the target data type.

By default, a cast can be invoked only by an explicit cast
request, that is an explicit CAST(x AS typename) or x::typename construct.

If the cast is marked AS ASSIGNMENT
then it can be invoked implicitly when assigning a value to a
column of the target data type. For example, supposing that
foo.f1 is a column of type text, then:

INSERT INTO foo (f1) VALUES (42);

will be allowed if the cast from type integer to type text is marked
AS ASSIGNMENT, otherwise not. (We
generally use the term assignment cast
to describe this kind of cast.)

If the cast is marked AS IMPLICIT
then it can be invoked implicitly in any context, whether
assignment or internally in an expression. (We generally use the
term implicit cast to describe this kind
of cast.) For example, consider this query:

SELECT 2 + 4.0;

The parser initially marks the constants as being of type
integer and numeric
respectively. There is no integer+numeric operator
in the system catalogs, but there is a numeric+numeric operator. The query will therefore succeed if
a cast from integer to numeric is available and is marked AS IMPLICIT — which in fact it is. The parser will
apply the implicit cast and resolve the query as if it had been
written

SELECT CAST ( 2 AS numeric ) + 4.0;

Now, the catalogs also provide a cast from numeric to integer. If that
cast were marked AS IMPLICIT — which it
is not — then the parser would be faced with choosing between the
above interpretation and the alternative of casting the
numeric constant to integer and applying the integer+integer operator. Lacking any knowledge of which
choice to prefer, it would give up and declare the query
ambiguous. The fact that only one of the two casts is implicit is
the way in which we teach the parser to prefer resolution of a
mixed numeric-and-integer expression as numeric;
there is no built-in knowledge about that.

It is wise to be conservative about marking casts as implicit.
An overabundance of implicit casting paths can cause PostgreSQL to choose surprising
interpretations of commands, or to be unable to resolve commands
at all because there are multiple possible interpretations. A
good rule of thumb is to make a cast implicitly invokable only
for information-preserving transformations between types in the
same general type category. For example, the cast from int2 to int4 can reasonably be
implicit, but the cast from float8 to
int4 should probably be assignment-only.
Cross-type-category casts, such as text to
int4, are best made explicit-only.

Note: Sometimes it is necessary for usability or
standards-compliance reasons to provide multiple implicit
casts among a set of types, resulting in ambiguity that
cannot be avoided as above. The parser has a fallback
heuristic based on type categories
and preferred types that can help to
provide desired behavior in such cases. See CREATE TYPE for more
information.

To be able to create a cast, you must own the source or the
target data type. To create a binary-coercible cast, you must be
superuser. (This restriction is made because an erroneous
binary-coercible cast conversion can easily crash the
server.)

Parameters

sourcetype

The name of the source data type of the cast.

targettype

The name of the target data type of the cast.

funcname(argtypes)

The function used to perform the cast. The function name
can be schema-qualified. If it is not, the function will be
looked up in the schema search path. The function's result
data type must match the target type of the cast. Its
arguments are discussed below.

WITHOUT FUNCTION

Indicates that the source type is binary-coercible to
the target type, so no function is required to perform the
cast.

WITH INOUT

Indicates that the cast is an I/O conversion cast,
performed by invoking the output function of the source
data type, and passing the result to the input function of
the target data type.

AS ASSIGNMENT

Indicates that the cast can be invoked implicitly in
assignment contexts.

AS IMPLICIT

Indicates that the cast can be invoked implicitly in any
context.

Cast implementation functions can have one to three arguments.
The first argument type must be identical to or binary-coercible
from the cast's source type. The second argument, if present,
must be type integer; it receives the type
modifier associated with the destination type, or -1 if there is none. The third argument, if
present, must be type boolean; it receives
true if the cast is an explicit cast,
false otherwise. (Bizarrely, the SQL
standard demands different behaviors for explicit and implicit
casts in some cases. This argument is supplied for functions that
must implement such casts. It is not recommended that you design
your own data types so that this matters.)

The return type of a cast function must be identical to or
binary-coercible to the cast's target type.

Ordinarily a cast must have different source and target data
types. However, it is allowed to declare a cast with identical
source and target types if it has a cast implementation function
with more than one argument. This is used to represent
type-specific length coercion functions in the system catalogs.
The named function is used to coerce a value of the type to the
type modifier value given by its second argument.

When a cast has different source and target types and a
function that takes more than one argument, it represents
converting from one type to another and applying a length
coercion in a single step. When no such entry is available,
coercion to a type that uses a type modifier involves two steps,
one to convert between data types and a second to apply the
modifier.

Notes

Remember that if you want to be able to convert types both
ways you need to declare casts both ways explicitly.

It is normally not necessary to create casts between
user-defined types and the standard string types (text, varchar, and char(n), as well as
user-defined types that are defined to be in the string
category). PostgreSQL provides
automatic I/O conversion casts for that. The automatic casts to
string types are treated as assignment casts, while the automatic
casts from string types are explicit-only. You can override this
behavior by declaring your own cast to replace an automatic cast,
but usually the only reason to do so is if you want the
conversion to be more easily invokable than the standard
assignment-only or explicit-only setting. Another possible reason
is that you want the conversion to behave differently from the
type's I/O function; but that is sufficiently surprising that you
should think twice about whether it's a good idea. (A small
number of the built-in types do indeed have different behaviors
for conversions, mostly because of requirements of the SQL
standard.)

Prior to PostgreSQL 7.3,
every function that had the same name as a data type, returned
that data type, and took one argument of a different type was
automatically a cast function. This convention has been abandoned
in face of the introduction of schemas and to be able to
represent binary-coercible casts in the system catalogs. The
built-in cast functions still follow this naming scheme, but they
have to be shown as casts in the system catalog pg_cast as well.

While not required, it is recommended that you continue to
follow this old convention of naming cast implementation
functions after the target data type. Many users are used to
being able to cast data types using a function-style notation,
that is typename(x). This notation is in fact nothing more
nor less than a call of the cast implementation function; it is
not specially treated as a cast. If your conversion functions are
not named to support this convention then you will have surprised
users. Since PostgreSQL allows
overloading of the same function name with different argument
types, there is no difficulty in having multiple conversion
functions from different types that all use the target type's
name.

Note: Actually the preceding paragraph is an
oversimplification: there are two cases in which a
function-call construct will be treated as a cast request
without having matched it to an actual function. If a
function call name(x) does not exactly match any existing
function, but name is the
name of a data type and pg_cast
provides a binary-coercible cast to this type from the type
of x, then the call will be
construed as a binary-coercible cast. This exception is made
so that binary-coercible casts can be invoked using
functional syntax, even though they lack any function.
Likewise, if there is no pg_cast
entry but the cast would be to or from a string type, the
call will be construed as an I/O conversion cast. This
exception allows I/O conversion casts to be invoked using
functional syntax.

Examples

To create an assignment cast from type bigint to type int4 using the
function int4(bigint):

CREATE CAST (bigint AS int4) WITH FUNCTION int4(bigint) AS ASSIGNMENT;

(This cast is already predefined in the system.)

Compatibility

The CREATE CAST command conforms to
the SQL standard, except that
SQL does not make provisions for binary-coercible types or extra
arguments to implementation functions. AS
IMPLICIT is a PostgreSQL
extension, too.